[A Case of Carcinoma Showing Thymus-Like Differentiation (CASTLE) of the Thyroid].

The case presented herein was a 70-year-old woman who had no compliant, but had a mass in the lower part of the right lobe of the thyroid detected by ultrasound (US). The US image of the tumor, measuring 13 mm in diameter, showed a low and heterogeneous internal echo level with calcification and an irregular margin. The tumor appeared to extend to the adjacent sternothyroid muscle, and cervical lymph node swelling was detected in a computer tomography (CT) image, but no metastatic lesion was found by positron emission tomography (PET)-CT. In a fine needle aspiration cytology of the tumor, papillary thyroid carcinoma was suggested because of the atypical epithelial cells having some changes other than intranuclear inclusion bodies. A subtotal thyroidectomy and central neck lymph node dissection were performed. The excised tumor was histologically composed of irregular nests or sheets of atypical squamoid epithelial cells with some ductal structures that leached to the sternothyroid muscle and involved the right lower parathyroid gland. Carcinoma showing thymus-like differentiation (CASTLE) was diagnosed histopathologically and immunohistochemically with the following immunohistochemical results: Cluster of differentiation 5 (CD5) (+), tumor protein p63 (p63) (+), KIT proto-oncogene receptor tyrosine kinase (c-KIT(CD117)) (+), thyroglobulin (-), and thyroid transcription factor-1 (TTF-1) (-). CASTLE is a rare carcinoma of the thyroid that architecturally resembles thymic epithelial tumors. Many CASTLE patients have been misdiagnosed as other carcinomas, such as anaplastic carcinoma, poorly differentiated carcinoma or squamous cell carcinoma of the thyroid. Immunohistochemical examination, including CD5 played an important role in the final diagnosis of CASTLE, although the distinction from diagnosis as squamous cell carcinoma or mucoepidermoid carcinoma in Hematoxylin-Eosin staining was challenging in our case. Nodal metastasis and perithyroidal tumor extension of CASTLE can predict its worse prognosis. Thus, at least careful follow-up studies are mandatory in cases of CASTLE.